There's a difference between selling at a loss, and having a loss.
OpenAI let's people use models for free with very little limits other than reducing the model quality over time, and they have very generous limits before they limit you at that.
That all costs money and is a loss for them.
If they get someone who's willing to pay, and they charge $20/m and on average, they net $5 profit per customer, they aren't selling it at a loss, they just need more customers. It's possible that a paid customer uses it even more though and it actually does incur a loss per paid customer and they're doing that to try and gain users while they figure out how to lower their costs, but that seems less likely.